This invention relates generally to friction brakes and more particularly concerns a spring-applied fail-safe brake actuator.
Friction brakes are used to slow or stop a machine by converting its energy of motion into heat. This is generally accomplished with a brake actuator which forces a friction element against a rotating braking surface to restrain its rotation. The present invention concerns an improved friction brake actuator for many applications and particularly for the heavy duty disc brake system of railroad vehicles and automotive trucks. The present invention would similarly be useful for actuating the brakes of aircraft and any mechanical devices requiring disc brakes such as large power hoists and the like.
A disc brake system generally consists of a brake disc gripped pincer-wise between a pair of brake pads fitted with friction linings. An operator-controlled brake actuator forces the friction linings of the brake pads into engagement with the brake disc to apply the brakes; and pulls the friction linings away to release the brakes.
The dependability of the disc brake system is a critical factor in the safe operation of a vehicle or machine on which the brakes are installed. In railroad vehicles and automotive trucks, the very lives of the operator and/or passengers may depend on the safe dependable operation of the brake system in potential accident situations. Furthermore, in urban commuter train networks brake dependability is one of the factors determinative of the required spacing between trains in the congested traffic on urban tracks. Improvements in brake dependability thereby allow such commuter trains to safely run closer together, faster or both, all of which results in more frequent commuter service for the urban population.
Accordingly, one object of the present invention is to improve the safety and dependability of friction brake systems, which object is accomplished by providing a fail-safe brake actuator which is effective to apply the brakes in response to brake system failure as hereinafter disclosed in detail.
Besides safety, the structure of friction brake actuators must satisfy additional conflicting criteria. Whereas in spring applied brakes, it is desirable to have the actuator incorporate a spring powerful enough for effective braking under the most severe conditions, it is also desirable to minimize the overall size of the spring and actuator. The problem is that as the brake pads wear, the spring expansion required per brake application increases to compensate for the lost pad thickness. In heavy duty actuators such as those used on commuter trains or trucks, a long spring with a lower spring rate would be required to provide a relatively constant braking force over the span of brake pad movement which varies according to pad wear. Such a spring, which could provide sufficient spring force would probably weigh seventy-five to one hundred pounds. Disc or Belleville springs are desirable for such actuators because of their extremely high load capacity but a Belleville spring system would have to be at least about two feet long to accomodate for pad wear. It is therefore a further object of the present invention to eliminate the effects of pad wear on the actuator spring so that a powerful spring expandable through a uniform short distance may be used to provide effective braking regardless of pad wear.
The structural criteria that actuators of commuter trains be of minimal size is largely due to the popularity of parallel motor drive. The power transmission efficiency and space savings of such an arrangement are well known. On railroad cars with right angle drive, wherein the power plant may be substantially removed from the drive axle and connected thereto by a drive shaft, it may be feasible to use a rather large cumbersome brake caliper. But in parallel drive units, wherein the power plant is positioned directly adjacent the drive axle, space is at a premium. It is therefore another object of the present invention to provide as small and compact a brake actuator as possible primarily by minimizing the length of spring expansion required to apply the brakes.
A similarly related object is to provide an actuator with a powerful but light spring for the consequent weight savings which is always a bonus in railroad and truck vehicle construction.
A further object is to eliminate the need for a separate and independent parking brake apparatus for vehicular brake systems. Specifically, it is an object to provide a spring-applied brake actuator which automatically performs as a parking brake.